Supercapacitors generally consist of the combination of two conductive electrodes having a high specific surface, immersed in an ionic electrolyte and separated by an insulating membrane, referred to as “separator”, which makes possible ionic conductivity and prevents electrical contact between the electrodes. Each electrode is in contact with a metal collector which makes possible exchange of the electric current with an external system. Under the influence of a difference in potential applied between the two electrodes, the ions present within an electrolyte are attracted by the surface exhibiting an opposite charge, thus forming an electrochemical double layer at the interface of each electrode. Electrical energy is thus stored electrostatically by separation of the charges.
The expression of the capacitance of such supercapacitors is identical to that of conventional electrical capacitors, namely:C=ε·S/t with: ε: the permittivity of the medium,
S: the surface area occupied by the double layer, and
t: the thickness of the double layer.
The capacitances achievable within supercapacitors are much greater than those commonly achieved by conventional capacitors, this being as a result of the use of porous electrodes having a high specific surface (maximization of the surface area) and of the extreme narrowness of the electrochemical double layer (a few manometers).
Moreover, the energy stored within the capacitor is defined according to the following expression:E=½·C·V2,in which V is the potential of the supercapacitance.
The capacitance and the potential are two essential parameters which it is necessary to optimize in order to promote the performances of the supercapacitors, the potential depending directly on the stability of the electrolyte under the influence of the electric field.
Thus, the electrodes used must necessarily be:                conducting, in order to provide for the transportation of the electric charges,        porous, in order to provide for the transportation of the ionic charges and the formation of the electrical double layer over a large surface area, and        chemically inert, in order to prevent any energy-consuming side reactions.        
Energy storage systems are thus particularly advantageous for applications requiring high powers while conveying significant energies. The possibilities of rapid charges and discharges, the increased lifetime with respect to a battery and the possibility of having systems based on non-toxic products make supercapacitors promising candidates for many applications.
Porous carbon-based materials, in the powder or monolith form, appear to be best suited to such applications. Among the porous carbon-based materials described in the prior art, carbon aerogels exhibit advantageous characteristics for supercapacitance applications due to their high porosity (R. W. Pekala et al., J. Mater. Sci., 24 (1989), 3221; C. Lin et al., Carbon, 35 (1997), 1271; B. Mathieu et al., Ann. Chim. Fr., 22 (1997), 19).
The specific surface of the carbon-based materials and the porosity actually accessible by the electrolyte are essential factors in the establishment and optimization of the electrochemical double layer. The resulting capacitance is commonly expressed with respect to the dry weight of the material. The term used is “specific capacitance”, expressed in F/g of dry carbon. Nevertheless, this method of calculation is not satisfactory insofar as it is not representative of the performances of the material when it is employed as electrode. A better balance between the quantitative numerical evaluation and the reality of the performance can be obtained by evaluation of the full capacitance by weight of the material, which takes into account the pore volume of this material. Maximizing the performance of the carbon-based electrodes ideally requires managing to increase this capacitance, which is a function of the accessible surface area, while reducing the pore volume of the materials. This is because this volume is occupied by the electrolyte (which increases the final weight of the electrodes), which lowers the full capacitance by weight (expressed in F/g of carbon filled with electrolyte). On considering that the two electrodes of the same system have the same specific capacitance, reference is made to “mean specific capacitance”.
WO 2009/125094 describes carbon-based materials resulting from the pyrolysis of resorcinol/formaldehyde latex (RFL) type, these materials exhibiting an adjusted porosity. However, the mean capacitances of these materials can still be improved.
Various chemical treatments which make it possible to enhance the capacitance performance of carbon-based materials have thus been described in the literature. They typically involve activation using CO2, HNO3, H2O2 or KOH (J. L. Figueiredo, Carbon, 37 (1999), 1379). In the majority of cases, these treatments consist in creating additional porosity by the local destruction of the carbon (C. Lin et al., Carbon, 38 (2000), 849). The disadvantage of this approach is the simultaneous increase in the capacitance and in the pore volume. The increase in the full specific capacitance (expressed in F/g of carbon filled with electrolyte) is thus not systematic since the weight of the material increases in parallel with the capacitance.
In addition, the activation treatment results in oxidation of the surface of the carbons, resulting in more or less significant grafting of oxygen-based functional groups exhibiting a redox activity (B. E. Conway, Electrochemical Supercapacitors—Scientific Fundamentals and Technological Applications, pp. 186-190). As the phenomena generated are faradaic and occur at the surface, they are fast and comparable to a capacitance contribution (reference is made to pseudocapacitance).
The presence of oxygen-based functional groups can also affect the wettability, indeed even the chemical and electrochemical reactivity, at the electrode/electrolyte interface and can thus promote the establishment of the electrochemical double layer (C. T. Hsieh, Carbon, 40 (2002), 667). However, the pseudocapacitance of such grafted materials still remains to be improved.
U.S. Pat. No. 5,993,996 relates to energy storage devices. This document describes a process for the treatment of porous carbon-based materials resulting from phenolic resins, the said process comprising a hydrogenation stage at a temperature of between 650 and 900° C. (this is a reduction stage intended to eliminate the oxygen-based functional groups at the surface of the carbon-based material), followed by a sulphonation stage carried out using a concentrated sulphuric acid solution at a temperature which can reach 290° C. Nevertheless, this process remains complex insofar as it necessarily comprises a preliminary hydrogenation stage.
Other documents of the prior art provide processes for grafting and for maximizing the contents of sulphur within porous carbon-based materials.
Baker et al. (W. S. Baker et al., J. Non-Cryst. Solids, 350 (2004), 80-87) describe in particular the modification of carbon-based surfaces by reacting resorcinol/formaldehyde (RF) gels with 3-thiophenecarboxaldehyde, the latter being inserted within the network after gelling the RF system. Thiophene groups are thus incorporated in the structure of the gel and result, after pyrolysis, in the appearance of residual sulphur-based functionalities. The major disadvantages of the materials resulting therefrom are their very low densities and capacitances by volume. Moreover, this process involves numerous stages and requires a very long implementation time (several days).
Zhang et al. (B. Zhang et al., Electrochimica Acta, 54 (2009), 3708-3713) describe the preparation of sulphur-based carbon-based materials obtained by heat treatment of mixtures of acetylene black and sulphur. This technique makes it possible to obtain graphitic carbon-based materials comprising a significant amount of sulphur (36% by weight). These materials (as a mixture with a binder of PTFE type) are used as cathodes in faradaic systems, i.e. Li—S batteries. In such systems, the diffusion of the entities is slow (limited overall kinetics of the electrode), which does not render them suitable for supercapacitance applications.
Valenzuela Calahorro et al. (C. Valenzuela Calahorro et al., Carbon, Vol. 28, Nos. 2/3, pp. 321-335, 1990) describe the introduction of sulphur into activated carbon-based materials using the gaseous agents H2S and SO2, according to different heating conditions. However, industrial processes based on the uses of such gases are toxic and remain complex to implement.
Lakshmi et al. (N. Lakshmi et al., J. Phys. D: Appl. Phys., 39 (2006), 2785-2790) describe carbon-based materials in the powder form intended to be used in fuel cells, the carbon-based materials being subjected to a treatment with ammonium sulphate at a temperature of 235° C., thus generating sulphur trioxide SO3. The latter subsequently reacts with the hydrogens located at the surface of the materials:(NH4)2SO4→2NH3+H2O+SO3 Carbon-H+SO3→Carbon-SO3H
The RF or RFL materials treated with ammonium sulphate (NH4)2SO4 nevertheless exhibit very low capacitances.